It definitely is an addition. In this case, this addition is clearly identifiable as part of the structure built after the original because the materials are different. The Secretary of the Interior Standard’s for Rehabilitation state that they must be distinguishable, so as to not to confuse people. Otherwise, we’d have too much fakery. Make sense?

Structurally it does what it needs to do – size and height are what is required..
So It is why the building is in use, not torn down. All Good.
The green color and vertical siding make it clear that it is not pretending to be original but is new. Also good, but jarring – red and green being opposite on the color wheel
I would have opted for a brown or grey – a color that was secondary to the brick, not competing with it.

I think it is imperative to have a clear delineation between old and new. I really dislike when they try to make something look like something historic;its dishonest and robs future generations of example of the best architectural styles of a particular time period. That being said, the design on this is ugly in my opinion.